Generally, an exhaust manifold refers to an exhaust pipe collecting exhaust gas discharged from each cylinder into one flow. The manifold may experience resistance because of differences in an internal diameter of a gasket, an internal diameter of a head, and an internal diameter of the manifold.
Since the exhaust manifold is placed at a site where an exhaust gas outputted from a cylinder head is first received, the exhaust manifold may be exposed to very high heat according to the power of an engine. Because there is no cooler such as cooling water in the exhaust manifold, unlike an engine having cooling water, when the engine is accelerated, the temperature may increase to about 800 to 900° C. from the high temperature exhaust gas and may then be rapidly air-cooled to normal temperature when the engine is stopped.
Since this process is repeated several times in one day, the heat impact on the exhaust manifold may be very severe, and thus high durability of the exhaust manifold among the various parts of the engine is desired.
A turbine housing is an external case of a turbo charger, and a turbine wheel and the like are in the turbine housing. Since the turbine housing is exposed to the high temperature of the exhaust gas outputted from the exhaust manifold, the turbine housing should have high durability like the exhaust manifold.
For high durability, a material used in the exhaust manifold and the turbine housing of a diesel engine, FCD-HS and SiMo cast irons and the like are used as a high-temperature oxidation-resistant cast iron. These materials are manufactured by adding an element such as silicon (Si) and molybdenum (Mo) to an existing nodular graphite cast iron material to improve physical properties and oxidation resistance at high temperatures.
However, a general-use temperature range of an exhaust system using heat-resistant cast iron is about 630 to 800° C., and in this temperature range, the aforementioned materials have tensile strength of about 60 MPa.
However, recently, due to the development of high-performance engines to meet the trend of increased output of vehicles, and also the strengthening of exhaust regulations, exhaust gas temperatures have increased. As the standard of durability and quality is strengthened, a load applied to the exhaust system is gradually increasing.
Therefore, the present disclosure has been made in an effort to develop a heat-resistant cast steel having superior strength and oxidation resistance at high temperatures to be used in an exhaust manifold and a turbine housing of a high performance engine.